Numerous commercial products are offered for smoke detection in small confined areas, such as rooms, and hallways in a house. They achieve performance according to published guidelines.
These smoke/fire detectors, however, are impractical in large areas with high ceilings, such as auditorium, theater, factory, and aircraft hangar, since these detectors are point sensors and detect smoke only in a small local vicinity to the detector. As a result, large numbers of these detectors are needed.
Installation on high ceilings is difficult. Furthermore, smoke may be dispersed and not reach the height of the ceiling to be detected. Projected and reflected beam smoke detectors, which predict the presence of smoke through measurements of the attenuation of a light beam, are possible solutions. However, in addition to having limited sensitivity, beam-based detectors require precise alignment between the source emitter and the light receiver. Hence such detectors are costly to install and maintain.
Various types of target based smoke detectors are known. Embodiments are disclosed in U.S. patent application Ser. No. 12/549,115 file on Aug. 27, 2009 entitled System and Method of Target Based Smoke Detection, assigned to the assignee hereof, and incorporated herein by reference.
Target-based smoke detection systems rely on the reduction in contrast of an observed target. Preferably the target provides areas that have crisp, sharp contrast under all operating conditions and environments. A good target design also enables the system to detect various types of smoke at different intensities. On the contrary, a poor target can cause an offset in contrast computation due to the quality of images captured by low cost cameras under low light or no light conditions rendering a degraded smoke detection performance.